The invention relates to an apparatus for radiation analysis by means of analyzing ionizing radiation, including a radiation detector for detecting the analyzing radiation, which detector includes:
a gas-filled absorption chamber for absorbing the radiation to be detected, which absorption chamber is provided with an entrance window which is formed in a wall of the absorption chamber and is transparent to the radiation to be detected, and
at least one counting wire which is arranged in the gas atmosphere, the surface of the entrance window being oriented transversely of the longitudinal direction of the counting wire.
The invention also relates to a radiation detector for use in such an apparatus.
A radiation detector for use in such an apparatus is described in U.S. Pat. No. 3,952,197. The radiation detector described therein includes a gas-filled, elongate chamber, the wall of which constitutes a first electrode. A rod-shaped or wire-shaped second electrode is arranged in a slit-shaped cut-out in the wall in such a manner that it extends parallel to the longitudinal direction of said chamber. A voltage difference exists between the two electrodes, so that a very inhomogeneous electrical field is present in the elongate chamber. The elongate chamber is closed at both its ends by end plates which extend transversely of the longitudinal direction and in which there is provided an entrance window which is permeable to the radiation to be detected.
A radiation detector for use in such an apparatus is described in U.S. Pat. No. 3,952,197. The radiation detector described therein includes a gas-filled, elongate chamber, the wall of which constitutes a first electrode. A rod-shaped or wire-shaped second electrode is arranged in a slit-shaped cut-out in the wall in such a manner that it extends parallel to the longitudinal direction of said chamber. A voltage difference exists between the two electrodes, so that a very inhomogeneous electrical field is present in the elongate chamber. The elongate chamber is closed at both its ends by end plates which extend transversely of the longitudinal direction and in which there is provided an entrance window which is permeable to the radiation to be detected.
In this known radiation detector the ion current to be detected is not amplified in the radiation detector itself. Consequently, for a given radiation intensity the current to be measured is very low or a very high intensity is required.
In radiation detectors of the kind generally known from prior art the problem imposed by an inadequate detection current is solved by producing an avalanche effect, i.e. the particles released upon ionization are accelerated by the electrical field, prior to collision with another gas particle, in such a manner that such a collision produces a new ionization; this process is repeated many times with the particles released by the new ionizations. The avalanche of released particles ultimately reaches the counting wire in which the large number of particles produces a current impulse which is much larger than that produced by a single particle.
Radiation detectors utilizing the avalanche effect, however, have the drawback that the shape of the current impulse is dependent on the location where the ionization, i.e. the beginning of the avalanche, occurs. This phenomenon is due to the fact that the incident X-ray quanta in such radiation detector require a long path through the gas so as to make the probability of ionization high enough for adequate X-ray detection. This means that ionizations occur both close to the counting wire as well as at a comparatively long distance therefrom. An ionization in the gas atmosphere of the detector causes a cloud of electrons whose size is dependent on the energy, i.e. the wavelength, of the X-rays to be detected. Such an ionization-induced cloud travels to the counting wire under the influence of the electrical field in the vicinity of this wire. While traveling to the counting wire the electrons of said cloud are driven apart from one another by mutual electrical repulsion, so that not only gas amplification of the current impulse occurs but also widening of this impulse. Consequently, ionization close to the counting wire produces a sharp impulse whereas, due to said repulsion, an ionization remote from the counting wire causes broadening of the impulse. Because the charge content of the impulse remains the same, the impulse is then also flattened proportionally. Consequently, it may occur that two wide impulses in rapid succession are not distinguished from one another but interpreted as a single impulse of higher energy by the processing electronics, thus leading to incorrect interpretation of the measurements. This problem can be circumvented by inhibiting the detection of a second impulse within a given period of time after a first detected impulse; this given period of time must then be chosen to be equal to the longest possible impulse duration. However, this makes the detector much slower and the duration of measurements will be prolonged proportionally.
It is an object of the invention to provide a radiation detector of the kind set forth which offers impulses of comparatively high current intensity without the counting speed of the radiation detector being degraded.
To this end, the apparatus for radiation analysis according to the invention is characterized in that the radiation detector is also provided with at least two avalanche chambers which adjoin the absorption chamber, are in atmospheric contact therewith and are intended to produce an avalanche of released charged particles, each of said avalanche chambers being provided with:
a counting wire which is present in the gas atmosphere,
a grid which is present in the gas atmosphere and is arranged around the counting wire,
the counting wires of said avalanche chambers extending substantially parallel to one another.
Because the surface of the entrance window is oriented transversely of the longitudinal direction of the counting wire, the radiation to be detected is incident substantially parallel to the counting wire in the absorption chamber. The absorption chamber can be constructed so as to have an arbitrary length within broad limits (i.e. the dimension in the direction parallel to the counting wire, so parallel to the incident radiation). Consequently, the absorption of the incident radiation can be proportionally high. The distance between the ionization and the counting wire may then be approximately constant and small, irrespective of the distance between the entrance window and the location of the ionization. The grids of the avalanche chambers are electrically adjusted relative to the absorption chamber in such a manner that an electron cloud formed in the absorption chamber will travel in the direction of the counting wires without the electrons in this cloud causing an avalanche of ionizations in the absorption chamber. Because of the small distance between each ionization and the grid, the electron cloud will hardly be dispersed during this short travel, so that no pulse broadening will be induced in this space. When the electron cloud enters the space between the grid and the counting wires (i.e. the avalanche chamber), it causes an avalanche of ionizations. This is due to the fact that the counting wires in the avalanche chambers are electrically adjusted relative to the grids in such a manner that an adequately strong electrical field is present in the avalanche chamber. Thus, for all electrons entering the avalanche chamber the avalanche commences at substantially the same distance from the counting wire. Because of the design of the avalanche chamber, this distance can be chosen so as to be sufficiently small to prevent broadening of the current impulse to be detected, so that the impulse duration is always short and hence the counting speed may be high.
Because several avalanche chambers are provided, the count rate of the detector may be higher than in the case of only one avalanche chamber. This increase is due to the fact that successive ionizations generally take place in different locations within the absorption chamber, so that the associated electron clouds will also travel to different avalanche chambers. A current impulse makes one avalanche chamber temporarily not accessible for a next impulse (the xe2x80x9cidle timexe2x80x9d), but another avalanche chamber can deal with an impulse. The effect of the idle time on the count rate of the detector is thus strongly reduced and may even become negligibly small when a sufficiently large number of avalanche chambers is used. Furthermore, because of the chosen construction of an absorption chamber, being separate from the avalanche chambers, the suspension of the counting wires may be such that the avalanche field generated in the avalanche chambers by these wires has an appearance which is not dependent on the location in the longitudinal direction where the electron enters the avalanche chamber. Consequently, this location does not influence the shape of the current impulse to be detected, so that the measuring result cannot be incorrectly interpreted.
Two types of gas-filled radiation detectors can be distinguished: so-called flow detectors and sealed detectors. The former type is used notably in the case of longwave X-rays. Because this type of radiation can be readily absorbed in an X-ray window, a very thin entrance window is used, often being a window made of a synthetic foil. Because such windows readily transmit the detector gas, gas is continuously supplied; this explains the name of these detectors. The latter type is used notably for shortwave X-rays and does not lose gas and hence is referred to as xe2x80x9csealedxe2x80x9d. It is to be noted that the invention can be used for both types of radiation detector.
The grid in an embodiment of the invention consists of grid wires which extend substantially parallel to the counting wire. This construction also results in an avalanche field which extends uniformly in the longitudinal direction of the counting wire and also enables a robust, comparatively vibration-insensitive suspension of the grid wires. Moreover, a desired cross-sectional shape can be readily imparted to the avalanche chambers bounded by the grid wires.
The avalanche chambers in a further embodiment of the invention directly adjoin one another. If the avalanche chambers were not to adjoin one another, areas in which the electrical field strength is substantially equal to zero would occur in the absorption chamber, so that the electron cloud released by the ionization would not travel in the direction of an avalanche chamber and hence would not be detected. By taking this step it can be ensured that the electron cloud formed in the absorption chamber due to ionization will always arrive in an avalanche chamber and hence will be detected. The probability of detection is thus significantly enhanced.
The avalanche chambers and the absorption chamber in a further embodiment of the invention constitute a contiguous stack. It is thus achieved that all electrons produced by ionization in the absorption chamber always reach an avalanche chamber, the dimensions of the detector nevertheless remaining limited. This can be achieved, for example by imparting a rectangular or square cross-section to the chambers. As a result of such a compact stacking, the individual avalanche chambers may have limited dimensions, offering the described advantages concerning the shape of the current impulse to be detected, a large volume being obtained nevertheless for the avalanche space. The uniformity of the avalanche field extending in the longitudinal direction of the counting wire is not affected by the square shape of the cross-section of the avalanche chambers. The rotational symmetry of the avalanche field, however, is slightly influenced by said, for example square shape, so that an electron entering at a corner of the square traverses a field other than an electron entering halfway the side of the square. However, the effect of this phenomenon is negligibly small for all practical purposes, because mainly the electrical field in the direct vicinity of the counting wire is of importance. The latter part of the field is hardly influenced by a square shape of the avalanche chamber.